R split data into 2 parts randomly

I am trying to split my data frame into 2 parts randomly. For example, I'd like to get a random 70% of the data into one data frame and the other 30% into other data frame. Is there a fast way to do this? The number of rows in the original data frame is over 800000. I've tried with a for loop, selecting a random number from the number of rows, and then binding that row to the first (70%) data frame using rbind() and deleting it from the original data frame to get the other (30%) data frame. But this is extremely slow. Is there a relatively fast way I could do this?